Add Xml.Escape helper method

[Copilot]

Adds a static Xml.Escape(string str) helper for producing valid XML text from arbitrary strings.

Changes

• PowerKit/Xml.cs — new Xml static class with Escape(string str):
  • Replaces the five XML special characters with their entities (&, <, >, ", ')
  • Removes characters illegal in XML 1.0 (control chars outside #x9/#xA/#xD, isolated surrogates, U+FFFE/U+FFFF)
  • Preserves valid supplementary characters (well-formed surrogate pairs)
  • Allocation-free fast path when the input requires no changes
• PowerKit.Tests/XmlTests.cs — tests covering each special character, control char removal, surrogate pair preservation, isolated surrogate removal, empty string, and null input

Example

Xml.Escape("1 < 2 & \"foo\" = 'bar'")
// → "1 &lt; 2 &amp; &quot;foo&quot; = &apos;bar&apos;"

Xml.Escape("foo\u0001bar")  // U+0001 is illegal in XML 1.0
// → "foobar"

[codecov]

Codecov Report

❌ Patch coverage is 89.74359% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.45%. Comparing base (ffbc21c) to head (2d8f6a7).

Files with missing lines	Patch %	Lines
PowerKit/Xml.cs	89.74%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #85      +/-   ##
==========================================
+ Coverage   85.31%   85.45%   +0.14%     
==========================================
  Files          70       71       +1     
  Lines        1137     1176      +39     
  Branches      202      218      +16     
==========================================
+ Hits          970     1005      +35     
  Misses        121      121              
- Partials       46       50       +4     

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[Copilot]

Pull request overview

Adds a small XML escaping utility to the PowerKit library (Xml.Escape(string)) to produce XML-safe text content by entity-escaping the five XML special characters and removing XML 1.0–illegal characters, with a no-allocation “no changes needed” fast path. Includes a focused test suite validating special-character escaping, invalid character removal, and surrogate handling.

Changes:

• Add PowerKit.Xml.Escape(string) to escape & < > " ' and remove XML 1.0–illegal characters (including isolated surrogates).
• Preserve valid supplementary characters via well-formed surrogate pairs.
• Add unit tests covering special chars, whitespace, control char removal, surrogate pairs, isolated surrogates, empty, and null.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
PowerKit/Xml.cs	Introduces Xml.Escape(string) with XML 1.0 character validity filtering and entity escaping.
PowerKit.Tests/XmlTests.cs	Adds unit tests validating escaping/removal rules and surrogate behavior.

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